The present invention relates to an image processing technology of detecting the position of an eye from a human face image for iris authentication and the like used for entrance/exit control and the like.
In recent years, the technology of personal authentication using iris recognition has been gradually brought into commercial use in association with entrance/exit control, automatic teller machine (ATM), and the like. As an example of such commercial use, conceived is a system of detecting the position of an eye from a human face image taken and performing iris recognition using an enlarged image of the detected eye position.
Techniques for detecting an eye from an image are disclosed in: (1) “Facial Feature Point Extraction Method Based on Combination of Shape Extraction and Pattern Matching”, The Institute of Electronics, Information and Communication Engineers, Transactions D-II, Vol. J80-D-II, No. 8, pp. 2170–2177, (2) Japanese Patent No. 2973676, (3) Japanese Laid-Open Patent Publication No. 10-63850, (4) Japanese Laid-Open Patent Publication No. 12-123148, (5) Japanese Laid-Open Patent Publication No. 8-185503, (6) Japanese Laid-Open Patent Publication No. 10-154220, for example.
(1) and (2) above disclose techniques for detecting an eye from an image taken under visible light. Specifically, the technique in (1) extracts features of an eye and the like from a gray-scale image of a face using a separability filter, a filter capable of locating a portion the center of which has a small brightness value while the periphery having a large brightness value. The technique in (2) extracts features of an eye and the like by subjecting a gray-scale image of a face to edge extraction filtering and matching the resultant edge image with templates of face components made of edges.
(3) to (6) disclose techniques for detecting an eye from an image taken by near infrared photography. Specifically, the techniques in (3) and (4) locate an eye by focusing attention on a change in the brightness of the eye in the vertical direction (upper eyelid→iris→lower eyelid). The techniques in (5) and (6) locate an eye by use of reflection of illumination light. More specifically, the technique in (5) utilizes the fact that in the case of co-axial lightning (in which the optical axis of an illuminator coincides with that of a camera), the brightness of reflection from the retina is different from that of reflection from the surface of the cornea, the rim and lenses of eyeglasses, and the like. That is, the eye position is determined from the reflection from the retina by appropriately setting two types of binary threshold values. The technique in (6) focuses attention on a plurality of kinds of reflection including reflection from the cornea of an eye. A kind of reflection exhibiting a large change in brightness with time is recognized as disappearance of reflection from the cornea due to blinking, and based on this recognition, the position of the eye is determined.
In iris recognition systems using the techniques described above, the illumination used is preferably near infrared light rather than visible light. The reason is that, in near infrared illumination, the subject of a photograph, who does not recognize the near infrared light, is prevented from being dazzled with the light and thus less feels a psychological repulsion against being photographed.
However, in an eye image taken under near infrared light, unlike an eye image taken under visible light, the iris has high reflectance and therefore only the pupil looks dark. In this case, therefore, the brightness becomes higher as the position shifts in the order of the pupil, the iris, the sclera, and the skin. This means that the difference in brightness between the iris and the sclera of the eye image is small compared with that of the image taken under visible light. Therefore, detection of an eye will not be successful when the technique of using gray-scale values as disclosed in (1) and the technique of using edge information obtained from gray-scale values as disclosed in (2) are directly applied to an eye image taken under near infrared light. This has been confirmed by experiments performed by the present inventors.
In the techniques in (3) and (4), attention is focused on a change in the brightness of an eye in the vertical direction because the change in the brightness of the eye in the lateral direction is small in an image taken under near infrared light. However, the brightness change of light→dark→light in the vertical direction also occurs at positions of the face other than the eyes, such as the eyebrows, the nostrils, and spots. There is therefore a great possibility of detecting a position other than the eyes by mistake.
The technique in (5) requires a special illuminator for co-axial lightning. In addition, this technique will not work well when the position of reflection from the lens or the like overlaps the position of reflection from the retina. In the technique in (6), it is necessary to wait for the subject blinking to observe a change with time. Therefore, it takes a certain amount of time to detect the eye position. Moreover, if the subject moves during this waiting time, detection of the eye may fail.